1. Field of the Invention
The present invention relates to an augmentation-index measuring apparatus which measures an augmentation index based on a cuff pulse wave occurring to a cuff worn on a living, subject.
2. Related Art Statement
Augmentation index, generally known as AI, indicates, e.g., a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, and is used to evaluate compliance of aorta. As the compliance of aorta increases, the reflected-wave component decreases and, as the compliance of aorta decreases, the reflected-wave component increases. More specifically described, if wall of aorta hardens, a reflected-wave component contained in the form of pulse wave obtained from the aorta increases. Thus, augmentation index reflects arteriosclerosis, and can be used as an index for inspecting arteriosclerosis.
As described above, augmentation index indicates a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, but it is difficult to separate a pulse wave detected (hereinafter, referred to as a detected pulse wave) into its incident-wave component and reflected-wave component. Hence, an augmentation index may be determined as follows: First, a detected pulse wave is analyzed to identify respective peak points of an incident-wave component and a reflected-wave component of the pulse wave. Then, the augmentation index is calculated by dividing a difference between a magnitude of the pulse wave at the time of occurrence of the peak of the incident-wave component and a magnitude of the pulse wave at the time of occurrence of the peak of the reflected-wave component, by a pulse pressure of the pulse wave. In addition, the peak of the incident-wave component may be determined as an inflection point or a local maximum point between a rising point of the detected pulse wave and a peak of the same; and the peak of the reflected-wave component may be determined as the first local maximum point following the peak of the incident-wave component.
Since augmentation index is used to evaluate compliance of aorta as described above, it is a clinical practice to non-invasively detect a pulse wave from a carotid artery that is the nearest to the aorta and determine an augmentation index based on the carotid pulse wave. However, first, it needs adequate skill to wear, at an appropriate position, a carotid-pulse-wave sensor for detecting a carotid pulse wave and, second, it is needed to use or employ the carotid-pulse-wave sensor. Thus, there is a need to easily measure an augmentation index using a cuff pulse wave which is detected from a cuff worn on, e.g., an upper arm of a living subject for measuring a blood pressure of the subject.
However, the cuff pulse wave occurring to the cuff pressing the portion of the living subject suffers the problem of low degree of reproducibility or stability, and accordingly the augmentation index obtained from the cuff pulse wave suffers the same problem and cannot enjoy high accuracy of measurement.
It is therefore an object of the present invention to provide an augmentation-index measuring apparatus which can measure an accurate augmentation index based on a cuff pulse wave.
The Inventors have performed extensive studies and found that since the skin and subcutaneous tissue of a portion of a living subject that is pressed by a cuff being wound therearound, e.g., the skin and subcutaneous tissue of an upper arm contain a high proportion of water, the portion is deformed by the pressing of the cuff in such a manner that the thickness of skin and subcutaneous tissue decreases, and that this is the cause of changing a relationship between the cuff and an artery located under the cuff and thereby time-wise changing the condition under which a cuff pulse wave is detected.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an augmentation-index measuring apparatus comprising a cuff which is adapted to be worn on a portion of a living subject to press the portion; an augmentation-index determining means for determining an augmentation index of the subject based on a cuff pulse wave obtained from the cuff and a preliminary pressing means for preliminarily pressing, before the cuff pulse wave is obtained, for the determination of the augmentation index, from the cuff having a pulse-wave detection pressure, the portion of the subject using the cuff having a pressure higher than the pulse-wave detection pressure.
According to this aspect of the invention, the preliminary pressing means preliminarily presses, before the cuff pulse wave is obtained for the determination of the augmentation index, the portion of the subject using the cuff having the pressure higher than the pulse-wave detection pressure used to press the portion for obtaining the cuff pulse wave. Therefore, the water contained in the skin and subcutaneous tissue of the portion being pressed by the cuff is squeezed out of the portion being pressed, so that the thickness of skin and subcutaneous tissue decreases to a value corresponding to the pressing. Since a cuff pulse wave is obtained in this state, the cuff pulse wave can enjoy a high stability, and an augmentation index determined based on the cuff pulse wave can enjoy a high accuracy.
According to a preferred feature of the first aspect of the present invention, the preliminary pressing means preliminarily presses the portion of the subject, within a sufficiently shorter time period than a time period in which the skin and subcutaneous tissue of the portion pressed by the means are restored to their initial state. For example, the preliminary pressing means finishes the preliminary pressing of the portion of the subject within several minutes (e.g., five or six minutes), more preferably, several tens of seconds (e.g., fifty or sixty seconds) before commencement of the obtaining of the cuff pulse wave for the determination of the augmentation index.
According to another feature of the first aspect of the present invention, the preliminary pressing means preliminarily presses the portion of the subject by carrying out a predetermined number of blood-pressure measurements using the cuff. According to this feature, the augmentation index can be obtained simultaneously with the blood-pressure measurement or measurements.
According to another feature of the first aspect of the present invention, the preliminary pressing means preliminarily presses the portion of the subject for a predetermined preliminary pressing time before the cuff pulse wave is obtained for the determination of the augmentation index. According to this feature, the portion of the subject is preliminarily pressed for the predetermined preliminary-pressing time, and accordingly the water contained in the skin and subcutaneous tissue of the portion being pressed by the cuff is squeezed out of the portion being pressed, so that the thickness of skin and subcutaneous tissue decreases to a value corresponding to the pressing of the cuff and, in this state, the cuff pulse wave is obtained for the determination of augmentation index.
According to another feature of the first aspect of the present invention, the preliminary pressing means preliminarily presses the portion of the subject with a pressure higher than a systolic blood pressure of the subject. According to this feature, the portion of the subject is preliminarily pressed with the pressure higher than the systolic blood pressure of the subject and accordingly the water contained in the skin and subcutaneous tissue of the portion being pressed by the cuff is squeezed out of the portion being pressed, so that the thickness of skin and subcutaneous tissue decreases to a value corresponding to the pressing of the cuff and, in this state, the cuff pulse wave is obtained for the determination of augmentation index.
According to another feature of the first aspect of the present invention, the augmentation-index measuring apparatus further comprises a cuff-pressure changing device which changes, for a blood-pressure measurement, the pressure of the cuff from a pressure higher than a systolic blood pressure of the subject to a pressure lower than a diastolic blood pressure of the subject, and the cuff pulse wave is obtained, immediately after the blood-pressure measurement, from the cuff having a pressure lower than the diastolic blood pressure of the subject obtained from the blood-pressure measurement. According to this feature, the augmentation index can be obtained simultaneously with the blood-pressure measurement.
A second aspect of the present invention is directed to use of the augmentation-index measuring apparatus according to the first aspect of the invention, for inspecting a degree of arteriosclerosis of the subject. That is, the augmentation-index measuring apparatus can be used as an arteriosclerosis-degree inspecting apparatus for inspecting the degree of arteriosclerosis of the subject based on the augmentation index determined by the augmentation-index determining means.